Digital broadcasting receiver

ABSTRACT

A digital broadcast recorder capable of recording a digital television (TV) signals, includes a first frequency converter which converts a received digitally modulated TV signal into a first intermediate frequency signal having a frequency lower than that of the digitally modulated TV signal, the first intermediate frequency signal having a frequency in a standard TV band and useable in demodulating standard TV signals, a band-pass filter coupled to the first frequency converter, which selectively passes-through the first intermediate frequency signal, a phase-lock loop (PLL) circuit coupled to the first frequency converter, which controls an oscillation frequency of a local oscillator in the first frequency converter, a second frequency converter coupled to the band-pass filter, which converts the first intermediate frequency signal passed through the band-pass filter into a second intermediate frequency signal lower in frequency than said first intermediate frequency signal a digital demodulator coupled to the second frequency converter, which demodulates the second intermediate frequency signal as converted by the second frequency converter and a writer coupled to the digital demodulator, which records demodulated signal as demodulated by said digital demodulator in a recording medium.

This is a continuation of application Ser. No. 08/695,507 filed Aug. 12,1996, now Pat. No. 5,956,098, which is a continuation of applicationSer. No. 08/386,038 filed Feb. 9, 1995, now U.S. Pat. No. 5,572,264.

BACKGROUND OF THE INVENTION

The present invention relates to a receiver which can receive a highdefinition TV signal and more particularly to a high definition TVsignal receiver which can receive a high definition TV signal which iscompressed to a band width which is the same as that of a TV signal ofthe standard system such as the NTSC system and a TV signal of thestandard system even when they are multiplexed and transmitted withinthe co-channel.

Furthermore, the present invention relates to a high definition TVsignal receiver which can selectively receive a high definition TVsignal and a standard TV signal and relates to a high definition TVsignal receiver which can share and receive a high definition TV signalwhich is compressed to a band width which is the same as that of a TVsignal of the standard system and a TV signal of the standard system (aTV signal of not only the NTSC system but also another standard systemwhich is modulated in amplitude is included).

As television broadcasting and CATV broadcasting have been expandedrecently, broadcasting channels are being multiplexed. In correspondencewith it, to reduce image interferences and reception interferences dueto leakage of a local oscillation signal even at the time ofmulti-channel reception, a receiver of the double super-heterodynesystem is used.

However, the aforementioned prior art is used to receive a TV signal ofthe standard system such as the NTSC system and reception of a signalwhich is digitally modulated such as a high definition TV signal is nottaken into account. Multiplexing and transmission of a TV signal of thestandard system and a high definition TV signal in the co-channel areneither taken into account.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a high definition TVsignal receiver which can selectively receive one of a TV signal of thestandard system and a high definition TV signal.

To accomplish the above object, the present invention uses the doublesuper-heterodyne system as a basic constitution so as to receive a highdefinition TV signal, uses a band pass filter having a flatness in passband characteristics and a small deviation of group delaycharacteristics which prevent the demodulation of a high definition TVsignal from degradation as a first intermediate frequency filter, uses ahigh definition TV signal filter (surface acoustic wave filter, etc.) asa second intermediate frequency filter, installs a third mixer forconverting a high definition TV signal to a third intermediate frequencysignal, and installs a high definition TV signal demodulator in thedemodulation section.

To share and receive a high definition TV signal and a TV signal of thestandard system, the present invention uses the double super-heterodynesystem as a basic constitution, uses a band pass filter having aflatness in pass band characteristics and a small deviation of groupdelay characteristics which prevent the demodulation of a highdefinition TV signal from degradation as a first intermediate frequencyfilter, uses a high definition TV signal filter and a filter (surfaceacoustic wave filter, etc.) for a TV signal of the standard system suchas a TV signal of the NTSC system as a second intermediate frequencyfilter, installs a third mixer for converting a high definition TVsignal to a third intermediate frequency signal, installs a highdefinition TV signal demodulator in the demodulation section, and an AMdemodulator for a TV signal of the standard system in the demodulationsection.

Furthermore, to share and receive a high definition TV signal and a TVsignal of the standard system, the present invention uses the doublesuper heterodyne system as a basic constitution, uses a band pass filterhaving a flatness in pass band characteristics and a small deviation ofgroup delay characteristics which prevent the demodulation of a highdefinition TV signal from degradation as a first intermediate frequencyfilter, uses a high definition TV signal filter as a second intermediatefrequency filter, installs a third mixer for converting a highdefinition TV signal to a third intermediate frequency signal, andinstalls a common demodulator for a high definition TV signal and a TVsignal of the standard system in the demodulation section.

The present invention having the aforementioned constitution canselectively select a TV signal of the standard system and a highdefinition TV signal even when they are multiplexed and transmitted inthe co-channel or even when they are transmitted independently.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the first embodiment of the highdefinition TV signal receiver of the present invention.

FIG. 2 is a block diagram showing the second embodiment of the highdefinition TV signal receiver of the present invention.

FIG. 3 is a block diagram showing the third embodiment of the highdefinition TV signal receiver of the present invention.

FIG. 4 is a block diagram showing the fourth embodiment of the highdefinition TV signal receiver of the present invention.

FIG. 5 is a block diagram showing the fifth embodiment of the highdefinition TV signal receiver of the present invention.

FIG. 6 is a block diagram showing the sixth embodiment of the highdefinition TV signal receiver of the present invention.

FIG. 7 is a block diagram showing the seventh embodiment of the highdefinition TV signal receiver of the present invention.

FIG. 8 is a block diagram showing the eighth embodiment of the highdefinition TV signal receiver of the present invention.

DETAILED DESCRIPTION

The embodiments of the present invention will be explained hereunderwith reference to the accompanying drawings. An example of a TV signalof the NTSC system (hereinafter referred to as just an NTSC signal) willbe used for explanation as a TV signal of the standard system.

FIG. 1 is a block diagram showing the first embodiment of the highdefinition TV signal receiver of the present invention. Numeral 10indicates a TV signal input terminal, 20 an input filter, 30 a variablegain amplifier, 40 a mixer, 50 an intermediate frequency filter, 60 anintermediate frequency amplifier, 70 a mixer, 80 an intermediatefrequency filter, 90 an intermediate frequency variable gain amplifier,100 a mixer, 110 an intermediate frequency filter, 120 an intermediatefrequency variable gain amplifier, 130 a local oscillator, 140 a PLLsynthesizer, 150 a standard oscillator, 160 a local oscillator, 170 aPLL synthesizer, 180 a standard oscillator, 190 a local oscillator, 200a change-over switch, 210 and 220 fixed value generation circuits, 230 acontrol circuit, 240 a channel selection signal input terminal, 250 anAGC change-over switch, 260 an amplification and smoothing circuit, 270a detection circuit, 280 a filter, 290 a high definition TV signaldemodulator, and 300 a high definition TV signal output terminal.

In the drawing, the digitized high definition TV signal is compressed indata, digitally modulated by QAM (quadrature amplitude modulation), QPSK(quadrature phase shift keying), or OFDM (orthogonal frequency divisionmultiplexing), and has a band width which is almost equal to that of anordinary TV signal. This high definition TV signal is inputted from theinput terminal 10 and divided into a VHF band and UHF band by the inputfilter 20 (furthermore, the VHF band may be divided into a low band,medium band, and high band) and only signals in the band including thedesired channel are extracted and supplied to the variable gainamplifier 30.

The input filter 20 is controlled in the control circuit 230 so as tobecome a suitable pass band by a channel selection signal which isinputted from the input terminal 240. The number of channels to besupplied to the variable gain amplifier 30 is reduced by the inputfilter 20. By doing this, mutual modulation interferences between inputsignals which are generated in the subsequent circuits can be reduced.

A signal whose band is restricted by the input filter 20 is amplified orattenuated to a suitable signal level by the variable gain amplitude 30and then supplied to the mixer 40.

On the other hand, the local oscillator 130 is controlled by the controlcircuit 230 and the PLL synthesizer 140 so as to oscillate at thefrequency corresponding to the desired channel by the channel selectionsignal which is inputted from the input terminal 240. The PLLsynthesizer 140 compares an oscillation signal having a stable frequencyfrom the standard oscillator 150 which is divided and an oscillationsignal from the local oscillator 130 which is divided and controls theoscillation frequency of the local oscillator 130 so that the errorthereof becomes zero. When the dividing ratio thereof is changedsuitably by the control circuit 230, the local oscillator 130 canoscillate at the frequency corresponding to the desired channel.

The mixer 40 mixes an output signal of the variable gain amplifier 30and a local oscillation signal from the local oscillator 130 and outputsa first intermediate frequency signal. The frequency of this firstintermediate frequency signal is set to more than the upper limit of theterrestrial broadcasting band of an NTSC signal or the CATV broadcastingband so as to reduce image interferences and to prevent receptioninterferences due to leakage of a local oscillation signal. For example,in the frequency allocation in Japan and USA, the 960 MHz band, 1200 MHzband, 1700 MHz band, 2600 MHz band, or 3000 MHz band is used.

The intermediate frequency filter 50 extracts only the desired channelof this intermediate frequency signal and supplies it to theintermediate frequency amplifier 60. The intermediate frequency filter50 is a band pass filter having a flatness in pass band characteristicsand a small deviation of group delay characteristics which prevent thedemodulation of a high definition TV signal from degradation and adielectric resonant filter or a surface acoustic wave filter is used.The output signal of the intermediate frequency filter 50 is amplifiedby the intermediate frequency amplifier 60, supplied to the mixer 70,and mixed with a local oscillation signal from the local oscillator 160so as to be switched to a second intermediate frequency signal.

The frequency of the second intermediate frequency signal is set to afrequency which is the same as that when a TV signal of the currentstandard system is received. Concretely, in USA, it is a 45 MHz bandwhich is the same as that when an NTSC signal is received. In Japan, itis a 58 MHz band.

The local oscillator 160 is controlled by the PLL synthesizer circuit170. The PLL synthesizer 140 compares an oscillation signal having astable frequency from the standard oscillator 180 which is divided andan oscillation signal from the local oscillator 16C which is divided andcontrols the oscillation frequency of the local oscillator 160 so thatthe error thereof becomes zero.

For the second intermediate frequency signal, only the desired channelis extracted by the intermediate frequency filter 80 and the signal isamplified or attenuated by the intermediate frequency variable gainamplification circuit 90 so as to be set to the desired signal level andthen inputted to the mixer 100.

The intermediate frequency filter 80 also requires a flatness in passband and a small deviation of group delay which prevent the demodulationcharacteristics of a high definition TV signal from degradation and itis necessary to eliminate interferences from the neighboring channel, sothat the intermediate frequency filter 80 consists of a surface acousticwave filter or others so as to pass only the band of the desiredreceiving channel.

The mixer 100 mixes an output signal of the intermediate frequencyamplifier 90 and a local oscillation signal from the local oscillator190 and outputs a third intermediate frequency signal. The thirdintermediate frequency signal is lower in frequency than the secondintermediate frequency signal and when the frequency almost coincideswith the symbol rate of a high definition TV signal, the demodulationcircuit is simplified. As described later, when the frequency band isset to a frequency band in which the FM modulation necessary to write ona recording medium such as a magnetic tape or optical disk can be easilyexecuted, the write circuit is simplified. When the frequency of thethird intermediate frequency signal is set low, various signalprocessing can be executed easily by using an operation amplifier whichcan process an analog signal with high precision as an amplifierinstalled on the subsequent stage of the mixer 100 or by converting ananalog signal to a digital signal.

The third intermediate frequency signal is supplied to the intermediatefrequency filter 110 and only a signal of the desired channel isextracted, amplified or attenuated by the intermediate frequencyvariable gain amplifier 120 so as to be set to the desired signal level,and then demodulated to a high definition TV signal by the highdefinition TV signal demodulator 290 and outputted from the outputterminal 300. An oscillation frequency fine tuning signal of the localoscillator 190 and a gain control signal of the intermediate frequencyvariable gain amplifier 120 are outputted from the high definition TVsignal demodulator 290.

An analog digital converter, which is not shown in the drawing, forconverting an analog signal to a digital signal between the intermediatefrequency variable gain amplifier 120 and the high definition TV signaldemodulator 290 may be installed. In this case, when the level of aninput signal of the analog digital converter is changed, thequantization error is changed and the normal operation of thedemodulator may be disturbed. However, since the intermediate frequencyvariable gain amplifier 120 is installed, the gain is finely tuned so asto keep the amplitude level unchanged.

When the power is turned on or the channel is changed, the operation ofthe high definition TV signal demodulator 290 is not stable, so that anoscillation frequency fine tuning signal of the local oscillator 190 anda gain control signal of the intermediate frequency variable gainamplifier 120 may not be as specified. Therefore, these control signalsare interrupted by the change-over switch 210 and in place of it, anoutput signal of the fixed value generation circuit 210 becomes a gaincontrol signal of the intermediate frequency variable gain amplifier 120and an output signal of the fixed value generation circuit 220 becomes afrequency fine tuning signal of the local oscillator 190.

When the channel is changed, the control circuit 230 sends a signalindicating that the channel is changed to the high definition TV signaldemodulator 290. By doing this, the high definition TV signaldemodulator 290 enters the initial state and the demodulation can startsmoothly.

A high definition TV signal outputted from the output terminal 300 issubjected to data decompression processing, and an image and voice areprocessed variously, and the signal is displayed on a display unit whichis not shown in the drawing.

An output signal of the intermediate frequency variable gain amplifier120 is also supplied to the filter 280 and only the desired band isextracted, detected by the detection circuit 270, and subjected tosuitable amplification and smoothing processing by the amplification andsmoothing circuit 260 so as to form a gain control signal. This gaincontrol signal is supplied to one of the variable gain amplifier 30 andthe intermediate frequency variable gain amplifier 90 as a gain controlsignal by the AGC change-over switch 250.

When the signal level of the third intermediate frequency signal fromthe intermediate frequency variable gain amplifier 120 is increased, again control signal which is obtained by the filter 280, the detectioncircuit 270, and the amplification and smoothing circuit 260 is suppliedto the intermediate frequency variable gain amplifier 90 by the AGCchange-over switch 250. After the gain attenuation amount of theintermediate frequency variable gain amplifier 90 is maximized, the AGCchange-over switch 250 supplies the gain control signal to the variablegain amplifier 30.

When the signal level of the third intermediate frequency signal isdecreased, the gain control signal is supplied to the variable gainamplifier 30 by the AGC change-over switch 250. After the gain of thevariable gain amplifier 30 is maximized, this gain control signal issupplied to the intermediate frequency variable gain amplifier 90 andthe gain of the intermediate frequency variable gain amplifier 90 isincreased.

By controlling the gain by the variable gain amplifier on the subsequentstage, the noise figure of the whole receiver can be prevented fromdegradation.

The gain control signal to the variable gain amplifier 30 is detected ata comparatively long time constant, and the gain control signal to theintermediate frequency variable gain amplifier 90 is set to acomparatively short time constant, and when the signal level is changedin a short period like a flutter by an airplane, the intermediatefrequency variable gain amplifier 90 on the subsequent stage follows thechange thereof.

The pass band of the filter 280 is set in the section in which the videocarrier on a high level of an NTSC signal which may be transmittedthrough the co-channel is excluded. By doing this, when a highdefinition TV signal is received, the gain change of the receiver due toan NTSC signal which is transmitted through the co-channel is reduced. Aband pass filter or notch filter is used for the filter 280.

For a high definition TV signal, the fact that it is transmitted throughthe co-channel along with an NTSC signal is taken into account. To avoidinterferences from the NTSC signal, the spectrum of high definition TVsignal is not arranged in the neighborhood of a video and an audiocarrier and a chromatic subcarrier on a high level of the NTSC signal.Therefore, it is necessary to install a notch filter for eliminating thecarrier and subcarrier of the NTSC signal in the high definition TVsignal demodulator 290.

As mentioned above, in this embodiment, to pass a high definition TVsignal through the intermediate frequency filters 50 and 80, a band passfilter having a flatness in pass band and a small deviation of groupdelay which prevent the demodulation characteristics of the highdefinition TV signal from degradation is used.

When the second intermediate frequency signal is converted to the thirdintermediate frequency signal having a lower frequency by the mixer 100,various signal processing can be executed easily by using an operationamplifier which can process an analog signal with high precision on thesubsequent stage of the mixer 100 or by converting an analog signal to adigital signal.

Furthermore, by using the filter 280 for eliminating the carrier part ona high level of a TV signal of the standard system such as an NTSCsignal which may be transmitted through the co-channel when a gaincontrol signal is detected, even when the two signals are transmittedthrough the co-channel, the gain control error can be reduced and thedesired high definition TV signal can be demodulated.

By doing this, even when a TV signal of the standard system such as anNTSC signal and a high definition TV signal are transmitted through theco-channel, the cross modulation by the two signals can be reduced andthe high definition TV signal can be demodulated at a low error rate.

FIG. 2 is a block diagram showing the second embodiment of the highdefinition TV signal receiver of the present invention. The same numeralis assigned to each part corresponding to FIG. 1 so as to omit duplicateexplanation. In this embodiment, the oscillation frequency of the localoscillator 160 is finely tuned by an oscillation frequency fine tuningsignal from the high definition TV signal demodulator 290.

FIG. 3 is a block diagram showing the third embodiment of the highdefinition TV signal receiver of the present invention. Numeral 131indicates a local oscillator, 141 a mixer, 151 a local oscillator, 161 aPLL synthesizer, and 171 a standard oscillator. The same numeral isassigned to each part corresponding to FIG. 1 so as to omit duplicateexplanation.

In the drawing, the first local oscillation signal from the localoscillator 131 and the second oscillation signal from the localoscillator 151 are mixed by the mixer 141 and the synthesizer compares asignal having the differential frequency thereof which is divided and anoscillation signal having a stable frequency from the standardoscillator 171 which is divided and controls the oscillation frequencyof the local oscillator 131 by the PLL synthesizer 161 so that the errorthereof becomes zero.

The local oscillator 131 is controlled by the control circuit 230 andthe PLL synthesizer circuit 161 so as to oscillate at the frequencycorresponding to the desired channel by a channel selection signalinputted from the input terminal 240. When the dividing ratio in the PLLsynthesizer circuit 161 is suitably changed by the control circuit 230,the local oscillator 131 can oscillate at the frequency corresponding tothe desired channel.

In this embodiment, in addition to the effect obtained in the firstembodiment shown in FIG. 1, the difference between the frequency of thefirst local oscillation signal and the frequency of the second localoscillation signal is kept constant in the co-channel and the number ofPLL synthesizers and standard oscillators can be reduced below thenumber in the first embodiment.

FIG. 4 is a block diagram showing the fourth embodiment of the highdefinition TV signal receiver of the present invention. The same numeralis assigned to each part corresponding to FIG. 1 so as to omit duplicateexplanation. In this embodiment, the oscillation frequency of the localoscillator 151 is finely tuned by an oscillation frequency fine tuningsignal from the high definition TV signal demodulator 290.

FIG. 5 is a block diagram showing the fifth embodiment of the highdefinition TV signal receiver of the present invention. Numeral 310indicates a recording medium and peripheral equipment thereof. The samenumeral is assigned to each part corresponding to FIG. 1 so as to omitduplicate explanation.

In the drawing, the recording medium and peripheral equipment thereof310 consists of a read/write magnetic disk or magnetic optical diskwhich can record a high definition TV signal or a phase conversionoptical disk and a drive circuit therefor.

For current standard TV signals such as NTSC signals, image and soundrecorders such as video tape recorders are now widespread. A programwhich is being viewed or a program which is not being viewed can berecorded while viewing the program and automatic recording can beexecuted by setting the time. Such recording requires partially changingthe base band signal of a standard TV signal such as an NTSC signal,modulating the frequency, and recording it on a magnetic tape. In thiscase, the signal in subjected to FM modulation at the carrier frequencywhich is decided from the relative speed between the magnetic tape andmagnetic head and the cap length of the magnetic head. Therefore, whenthe frequency of the third intermediate frequency coincides with afrequency which can be easily modulated, the signal processing unitwhich is necessary for writing can be simplified. In this case, an FMmodulator is installed in the input section of the recording medium andperipheral equipment thereof 310 and an FM demodulator is installed inthe output section thereof.

A high definition TV signal has a large amount of information comparedwith a standard TV signal. However, the frequency band width of the baseband signal of the high definition TV signal is the same as that of thestandard TV signal due to highly efficient image coding and entropycoding. Therefore, when the high definition TV signal is recorded beforeit is demodulated to digital data, the efficiency which is almost equalto that of the standard TV signal can be obtained from a viewpoint ofstorage capacity and images can be recorded for many hours.

When the aforementioned analog image recording mode is added to a highdefinition TV signal image recorder so as to record images for manyhours in addition to the digital image recording mode for recordingdigital information as it is, it is convenient to recording of imagesduring absences. In the case of analog recording, image degradation maybe caused by a temperature change or a change with time of a magnetictape, magnetic optical disk, or phase conversion optical disk. However,a high definition TV signal is digital information, so that it can becorrected by the error correction technique and the image quality whichis equivalent to that of the digital image recording mode can beobtained.

FIG. 6 is a block diagram showing the sixth embodiment of the highdefinition TV signal receiver of the present invention. Numeral 31indicates a variable attenuator, 32 an amplifier, 33 a variableattenuator, and 251 an AGC change-over switch. The same numeral isassigned to each part corresponding to FIG. 1.

In the drawing, a TV signal inputted from the input terminal 10 issupplied to the input filter 20 and the band including the desiredchannel is extracted. An output signal of this input filter 20 issuitably amplified or attenuated by the variable attenuator 31, theamplifier 32, and the variable attenuator 33 and inputted to the mixer40.

When the level of the input signal is increased, the AGC change-overswitch 251 switches and controls a gain control signal from theamplification and smoothing circuit 260 so that the gain attenuationoperation starts from the intermediate frequency variable gain amplifier90 in correspondence with it, and the variable attenuator 33 startsattenuation by leaving the attenuation amount corresponding to the levelchange such as the aforementioned flutter, and the variable attenuator31 starts attenuation at the point of time that the attenuation amountis maximized. When the input level is decreased, the AGC change-overswitch 251 switches and controls the gain control signal so that theattenuation amount of the variable attenuator 31 is decreased incorrespondence with it, and next the attenuation amount of the variableattenuator 33 is decreased, and finally the gain of the intermediatefrequency variable gain amplifier 90 is increased.

When the AGC change-over switch 251 is operated as mentioned above, thenoise figure of the whole receiver can be prevented from degradation. Atthe time of gain attenuation, the degradation of the noise figure can bereduced lower than that in the first embodiment shown in FIG. 1.

Since the variable attenuator is divided and each is delayed andoperated, the sensitivity of gain control amount to the AGC voltage canbe reduced and a stable AGC operation can be obtained.

Furthermore, when the variable attenuator 31 is installed on thepreceding stage of the amplifier 32, even when a TV signal on themaximum input level is inputted to the input terminal 10, mutualmodulation interferences and cross modulation interferences can bereduced.

FIG. 7 is a block diagram showing the seventh embodiment of the highdefinition TV signal receiver of the present invention. Numeral 81indicates an NTSC signal intermediate frequency filter, 91 an NTSCsignal intermediate frequency variable gain amplifier, 291 an NTSCsignal demodulator, 320 a change-over switch, and 330 an NTSC signaloutput terminal. The same numeral is assigned to each part correspondingto FIG. 1.

Also illustrated in FIG. 7 is the A/D converter 125 described above inconnection with FIG. 1, but not shown. The A/D converter, as previouslydescribed, is between amplifier 120 and demodulator circuit 290.

In this embodiment, an NTSC signal and a high definition TV signal canbe selectively received.

Namely, in FIG. 7, even when an input TV signal from the input terminal10 is an NTSC signal which is modulated in amplitude, or a highdefinition TV signal, or a signal in which an NTSC signal which ismodulated in amplitude and a high definition TV signal are multiplexedthrough the co-channel, according to the channel selected by a viewer,when the channel is an NTSC signal, it is demodulated by the NTSC signaldemodulator 291 and outputted from the NTSC signal output terminal 330.When the selected channel is a high definition TV signal, it isdemodulated by the high definition TV signal demodulator 290 andoutputted from the high definition TV signal output terminal 300.

When a signal in which an NTSC signal which is modulated in amplitudeand a high definition TV signal are multiplexed through the co-channelis inputted, it is demodulated by the high definition TV signaldemodulator 290 and outputted from the high definition TV signal outputterminal 300. When the high definition TV signal cannot be received dueto the Cliff effect in this case, the NTSC signal in the co-channel isautomatically demodulated by the NTSC signal demodulator 291 andoutputted from the NTSC signal output terminal 330.

Therefore, the intermediate frequency filter 50 is shared by a highdefinition TV signal, and an NTSC signal and a band pass filter having aflatness in pass band and a small deviation of group delay which preventthe demodulation characteristics of a high definition TV signal fromdegradation is used.

The second intermediate frequency signal is branched by the receptionchange-over switch 320 according to the high definition TV signal or theNTSC signal and supplied to the high definition TV signal intermediatefrequency filter 80 which consists of an SAW filter or others or theNTSC signal intermediate frequency filter 81 and only the band of thedesired receiving channel passes.

When a high definition TV signal is received, it is demodulated by anoperation which is the same as that in the embodiment shown in FIG. 1.

On the other hand, when an NTSC signal is received, the secondintermediate frequency signal is amplified by the NTSC signalintermediate frequency variable gain amplifier 91 and supplied to theNTSC signal demodulator 291 so as to be demodulated to an NTSC signal.This NTSC signal consists of video and audio signals of the base bandand outputted from the NTSC signal output terminal 330.

When a high definition TV signal is received, the change-over switch 320is switched to the high definition TV signal receiving side, and thethird intermediate frequency signal is branched in the same way as withthe embodiment shown in FIG. 1, and only the desired band in the thirdintermediate frequency signal is extracted by the filter 280, and thesignal level thereof is detected by the detection circuit 270 andsuitably amplified and smoothed by the amplification and smoothingcircuit 260 so as to form a gain control signal. This gain controlsignal is supplied to the variable gain amplifier 30 or the variablegain amplifier 90 via the AGC change-over switch 250.

When an NTSC signal is received, the change-over switch 320 is switchedto the NTSC signal receiving side and a gain control signal which isgenerated in the NTSC signal demodulator 291 is supplied to the variablegain amplifier 30 or the variable gain amplifier 90 via the AGCchange-over switch 250.

When a high definition TV signal is received, an oscillation frequencyfine tuning signal is supplied to the local oscillator 190 from the highdefinition TV signal demodulator 290. However, when an NTSC signal isreceived, a frequency fine tuning signal is supplied to the controlcircuit 230 from the NTSC signal demodulator 291 and by doing this, thecontrol circuit 230 controls so as to finely tune the oscillationfrequency of the local oscillator 160.

To reduce the power consumption, it is possible to supply power only toone of the high definition TV signal demodulator 290 and the NTSC signaldemodulator 291 according to the received signal.

As explained above, in this embodiment, not only a high definition TVsignal but also an NTSC signal can be received and the scale can bereduced by allowing these signals to share a part of the circuit.

FIG. 8 is a block diagram showing the eighth embodiment of the highdefinition TV signal receiver of the present invention. Numeral 292indicates a common demodulation circuit of a high definition TV signaland an NTSC signal, 301 an NTSC signal output terminal, and 321 achange-over switch. The same numeral is assigned to each partcorresponding to FIG. 1.

In this embodiment, an NTSC signal and a high definition TV signal canbe selectively received in the same way as with the embodiment shown inFIG. 7.

When a high definition TV signal is received in FIG. 8 the change-overswitch 321 is switched to the high definition TV signal receiving side.By doing this, the constitution becomes equal to that in the firstembodiment shown in FIG. 1 and the same receiving operation isperformed.

When an NTSC signal is received, the change-over switch 321 is switchedto the NTSC signal receiving side. The intermediate frequency 80 is fora high definition TV signal and the NTSC signal intermediate frequencyfilter 81 having a special frequency characteristics as shownembodiments in FIG. 7 is not used. Therefore, the embodiment is notsuited to handling of an AM modulation signal of the vestigial side-bandsystem such as an NTSC signal.

When an NTSC signal is received, the common demodulation circuit 292 ofa high definition TV signal and an NTSC signal generates a base bandsignal of the NTSC signal using a digital filter. For that purpose, bysharing the digital signal processing operation circuit and memory whichare installed for demodulation, data decompression, and image processingof a high definition TV signal partially, the circuit scale can bereduced. This digital filter is a band pass filter showing a frequencycharacteristic which is the same as that of the NTSC signal intermediatefrequency filter 81 but the center frequency is different from eachother.

When a signal is received in which an NTSC signal and a high definitionTV signal are multiplexed through the co-channel, the change-over switch321 is switched to the high definition TV signal receiving side and thesame receiving operation as that in the first embodiment shown in FIG. 1is performed. When the high definition TV signal cannot be received dueto the Cliff effect, the change-over switch 321 is changed to the NTSCsignal receiving side so as to automatically receive the NTSC signal inthe co-channel.

By doing this, when broadcasting by a high definition TV signal isexactly equal in content to broadcasting by an NTSC signal through theco-channel, even when the high definition TV signal cannot be received,the program during viewing can be continued with pleasure withoutstopping.

In this case, when the screen frame by the high definition TV signal isperfectly synchronized with the screen frame by the NTSC signal andvoice is continued, it is more convenient. Furthermore, in this case,when the horizontal and vertical resolutions of the NTSC signal are madeequivalent to those of the high definition TV signal by performing theinterpolation processing, the unpleasant feeling when a signal isswitched can be reduced.

When it is decided whether there is an error between the screen frame bya high definition signal and the screen frame by an NTSC signal on thebroadcasting station side or when any signal for synchronizing thescreens is added from the broadcasting station side, by delaying one ofthe signals for the error, for example, by the memory, they can besynchronized with each other.

When no consideration is given to the aforementioned interframe error bythe broadcasting station side, image comparison imposes an extremelyheavy burden on the hardware, so that voice of a high definition TVsignal and voice of an NTSC TV signal are compared with each other andthe magnitude of frame error is calculated. For this calculation, notonly voice but also the vertical synchronizing signal of the highdefinition TV signal and NTSC signal are used. Namely, by using thevertical synchronizing signal, the voice amplitude for each field (oreach frame) or the spectrum after high speed Fourier conversion iscompared between the high definition TV signal and NTSC signal and thefields (frames) having the greatest correlation are inferred as the samefield (frame). By adding the difference in processing time between ahigh definition TV signal and an NTSC signal in a television set as aparameter beforehand, the number of fields (frames) to be compared isdecreased and a result can be inferred more rapidly. This processing maybe performed once when the channel is switched.

When voice is continued as mentioned above, a high definition TV signaland an NTSC signal can be synchronized with each other.

As explained above, in this embodiment, not only a high definition TVsignal but also an NTSC signal can be received. Furthermore, when a highdefinition TV signal and an NTSC signal are multiplexed through theco-channel, even when the high definition TV signal becomesunreceivable, a function for automatically receiving the NTSC signal canbe added.

When an NTSC signal is digitally demodulated and digitally subjected toimage and voice processing, almost all the circuits can be shared by ahigh definition TV signal and an NTSC signal, and the circuit scale canbe reduced substantially and can be made adjustment free, and a standardTV signal (the PAL system or SECAM system) other than an NTSC signal canbe processed by changing the software without adding or changing thehardware.

Furthermore, even when the conventional NTSC signal software resources(video of a movie, a product created by an individual using a videomovie, etc.) are incorporated into a computer and any processing isadded, an NTSC signal is handled as a digital signal in a televisionset, so that it is possible only by installing any output terminal (anSCSI interface, etc.).

The effects shown in the embodiments in FIGS. 2, 3, and 4 can be addedto each of the embodiments shown in FIGS. 5, 6, 7, and 8. Furthermore,the effect shown in the embodiment in FIG. 6 can be added to each of theembodiments shown in FIGS. 7, and 8.

As explained above, according to the present invention, when the firstintermediate frequency filter is shared by a TV signal of the standardsystem and a high definition TV signal and the second intermediatefrequency filter and demodulator are installed in the TV signal of thestandard system and high definition TV signal individually, both the TVsignal of the standard system and high definition TV signal can beselectively received. Also the second mixer of the doublesuper-heterodyne system can be shared by these TV signals. Thus, thecircuit scale can be reduced.

According to the present invention, when a high definition TV signalreceived is converted to the third intermediate frequency signal with afrequency lower than that of the second intermediate frequency signal bythe third mixer, various signal processing can be executed easily byusing an operation amplifier which can process an analog signal withhigh precision as an amplifier installed on the subsequent stage of thethird mixer or by converting an analog signal to a digital signal. Evenwhen a TV signal of the standard system and a high definition TV signalare multiplexed and transmitted through the co-channel, the crossmodulation by these TV signals can be reduced and hence a highdefinition TV signal at a low error rate can be demodulated.Furthermore, according to the present invention, when a high definitionTV signal is converted to the aforementioned third intermediatefrequency signal, the frequency of signals handled by the highdefinition TV signal demodulator is reduced, so that the circuitconstitution is simplified.

Furthermore, according to the present invention, when a TV signal of thestandard type is digitally demodulated, a receiver shared by a smallamount of general purpose high definition TV signals and TV signals ofthe standard system of the analog circuit can be configured. When a TVsignal of the standard system and a high definition TV signal aremultiplexed and transmitted through the co-channel in this case, evenwhen the high definition TV signal becomes unreceivable, the receivercan be automatically switched so as to receive the TV signal of thestandard system.

To detect the gain control signal of the first variable gain amplifieror the second intermediate frequency variable gain amplifier, when a TVsignal of the standard system and a high definition TV signal aremultiplexed and transmitted through the co-channel, by eliminating thecarrier part with high energy in the TV signal of the standard system bya filter, an error operation of gain control by the TV signal of thestandard system can be avoided when a high definition TV signal isreceived and the high definition TV signal can be demodulatedsatisfactorily.

What is claimed is:
 1. A digital broadcast receiver capable of receivingdigitally modulated television (TV) signals, said receiver comprising: afirst gain amplifier, which amplifies a received digitally modulated TVsignal; a first frequency converter adapted to convert the digitallymodulated TV signal, which is amplified by said first gain amplifierinto a first intermediate frequency signal lower in frequency than saidreceived signal; an extractor, which extracts a signal corresponding toa specific channel in a predetermined frequency band; a second frequencyconverter adapted to convert the first intermediate frequency signalpassed through said extractor into a second intermediate frequencysignal lower in frequency than said first intermediate frequency signal;a digital demodulator adapted to demodulate the second intermediatefrequency signal as converted by said second frequency converter; and again controller, which controls gain of said first gain amplifiercorresponding with an output signal of said extractor.
 2. A digitalbroadcast receiver capable of receiving digitally modulated television(TV) signals, said receiver comprising: a first gain amplifier, whichamplifies a digitally modulated TV signal received; a first frequencyconverter adapted to convert the digitally modulated TV signal, which isamplified by said first gain amplifier into a first intermediatefrequency signal lower in frequency than said received signal; anextractor, which extracts a signal corresponding to a specific channelin a predetermined frequency band; a second frequency converter adaptedto convert the first intermediate frequency signal passed through saidextractor into a second intermediate frequency signal lower in frequencythan said first intermediate frequency signal; a digital demodulatoradapted to demodulate the second intermediate frequency signal asconverted by said second frequency converter; a gain controller, whichcontrols gain of said first gain amplifier corresponding with an outputsignal of said extractor; a third frequency converter adapted to convertthe digitally modulated TV signal, which is amplified by said first gainamplifier into a third intermediate frequency signal higher in frequencythan said received signal; and wherein said signal converted by saidthird frequency converter is input into said first frequency converter.3. A digital broadcast receiver capable of receiving digitally modulatedtelevision (TV) signals, said receiver comprising: a first gainamplifier, which amplifies a received digitally modulated TV signal; afirst frequency converter adapted to convert the digitally modulated TVsignal which is amplified by said first gain amplifier into a firstintermediate frequency signal in a transmission band; an extractor,which extracts a signal in predetermined frequency band; a second gainamplifier, which amplifies said extracted signal; a second frequencyconverter adapted to convert the intermediate frequency signal passedthrough said second amplifier into a second intermediate frequencysignal lower in frequency than said first intermediate frequency signal;a digital demodulator adapted to demodulate the second intermediatefrequency signal as converted by said second frequency converter; and again controller, which controls gain of said first gain amplifier andgain of said second gain amplifier corresponding with an output signalof said extractor.
 4. A digital broadcast receiver according to claim 3,further comprising: a third frequency converter adapted to convert thedigitally modulated TV signal, which is amplified by said first gainamplifier into a third intermediate frequency signal higher in frequencythan said received signal; wherein said signal converted by said thirdfrequency converter is input into said first frequency converter.
 5. Adigital broadcast receiver capable of receiving digitally modulatedtelevision (TV) signals, said receiver comprising: a first gainamplifier which amplifies a received digitally modulated TV signal; afirst frequency converter adapted to convert the digitally modulated TVsignal, which is amplified by said first gain amplifier into a firstintermediate frequency signal in a transmission band; an extractor,which extracts a signal in predetermined frequency band; a second gainamplifier for amplifying said extracted signal; a second frequencyconverter adapted to convert the intermediate frequency signal passedthrough said second amplifier into a second intermediate frequencysignal lower in frequency than said first intermediate frequency signal;a digital demodulator adapted to demodulate the second intermediatefrequency signal as converted by said second frequency converter; a gaincontroller, which controls gain of said first gain amplifier and gain ofsaid second gain amplifier corresponding with an output signal of saidsecond frequency converter.
 6. A digital broadcast receiver according toclaim 5, further comprising: a third frequency converter adapted toconvert the digitally modulated TV signal, which is amplified by saidfirst gain amplifier into a third intermediate frequency signal higherin frequency than said received signal; wherein said signal converted bysaid third frequency converter is input into said first frequencyconverter.
 7. A digital broadcast receiver according to claim 5, furthercomprising: a switch for switching which switches said gain controllerto control said first gain amplifier or to control said second gainamplifier.
 8. A digital broadcast receiver according to claim 5, whereinsaid gain controller controls said first gain amplifier first, thencontrols said second gain amplifier when the output signal level of saidsecond frequency converter is increased.
 9. A digital broadcast receiveraccording to claim 5, wherein said gain controller controls said secondgain amplifier first, then controls said first gain amplifier when theoutput signal level of said second frequency converter is decreased. 10.A digital broadcast receiver according to claim 5, wherein said gaincontroller controls said gain amplifier by a gain control signal, andsaid signal for said first gain amplifier has a longer time constantthan for said second gain amplifier.
 11. A digital broadcast receiveraccording to claim 5, wherein said first intermediate frequency signalhas a frequency in a standard TV band and useable in demodulatingstandard TV signals.
 12. A digital broadcast receiver according to claim5, further comprising: a display, which displays a signal outputted bysaid digital demodulator.
 13. A digital broadcast receiver according toclaim 1, wherein said first intermediate frequency signal has afrequency in a standard TV band and usable in demodulating a standard TVsignal.
 14. A digital broadcast receiver according to claim 2, whereinsaid first intermediate frequency signal has a frequency in a standardTV band and usable in demodulating standard TV signal.
 15. A digitalbroadcast receiver according to claim 3, wherein said first intermediatefrequency signal has a frequency in a standard TV band and usable indemodulating standard TV signal.
 16. A digital broadcast receiveraccording to claim 1, wherein said extractor has a SAW filter.